From models to materials (bonding continuum, alloys, polymers) practice questions.

The bonding triangle (van Arkel–Ketelaar diagram) places compounds according to average electronegativity and electronegativity difference. Where would a compound with a very high average electronegativity and a very small electronegativity difference be located?

• Pure metallic bonding corner
• Pure ionic bonding corner
• Pure covalent (molecular) bonding corner
• The centre, showing equal ionic, metallic and covalent character

Answer: Pure covalent (molecular) bonding corner

High average electronegativity with a small electronegativity difference means two non-metals of similar electronegativity, so the bonding is predominantly covalent (bottom-right corner of the data-booklet triangle). A large difference would place it at the ionic (top) corner instead.

Identify the structure and bonding in silicon dioxide, SiO2, which has a very high melting point and is very hard.

• Discrete covalent molecules held by London forces
• A giant covalent (network) structure
• A metallic lattice of delocalised electrons
• An ionic lattice of Si4+ and O2- ions

Answer: A giant covalent (network) structure

SiO2 forms a giant covalent network in which every atom is covalently bonded into a continuous lattice, so many strong bonds must break to melt it.

An alloy is generally harder and less malleable than the pure metal from which it is made. Explain which factor is primarily responsible.

• The added atoms greatly increase the number of delocalised electrons in the metallic lattice
• Atoms of different sizes disrupt the regular lattice, hindering the layers from sliding
• The added atoms form covalent bonds that lock the structure
• The alloy contains ionic bonds absent from the pure metal

Answer: Atoms of different sizes disrupt the regular lattice, hindering the layers from sliding

Alloying introduces atoms of a different size that distort the orderly lattice, so the layers of metal ions can no longer slide past one another easily, making the alloy harder.

Which property is best explained by the model of positive metal ions in a 'sea' of delocalised electrons?

• The brittleness of ionic solids
• The electrical conductivity of a metal in the solid state
• The unusually low melting point of simple molecular substances
• The insulating behaviour of diamond

Answer: The electrical conductivity of a metal in the solid state

Delocalised electrons in a metallic lattice are free to move throughout the structure, allowing metals to conduct electricity even as solids.

Polyethene (polythene) is formed from ethene monomers. Determine the type of polymerisation and the repeating unit.

• Condensation polymerisation, releasing water
• Addition polymerisation, repeating unit -CH2-CH2-
• Condensation polymerisation, repeating unit -CH2-CH2-
• Addition polymerisation, releasing HCl

Answer: Addition polymerisation, repeating unit -CH2-CH2-

Ethene is an alkene, so its C=C double bond opens and monomers add together with no small molecule lost; the repeating unit is -CH2-CH2-.

Which statement correctly distinguishes condensation polymerisation from addition polymerisation?

• Addition polymers eliminate a small molecule such as water; condensation polymers do not
• Condensation polymerisation eliminates a small molecule; addition polymerisation does not
• Both types eliminate water for every bond formed
• Only addition polymers can be formed from monomers with C=C bonds and always release HCl

Answer: Condensation polymerisation eliminates a small molecule; addition polymerisation does not

Condensation polymerisation joins monomers with loss of a small molecule (e.g. water), whereas addition polymerisation joins unsaturated monomers with no atoms lost.